This invention relates to the field of thermal neutron flux detection and measuring apparatus.
An electret is a dielectric material that is possessed of a quasi-permanent electrostatic polarity ---- a polarity in which the static electric potential at one surface is greater than at the other. This condition can be produced by the preferential orientation of dipolar molecules within the material, and by the trapping of electrons or ions near one surface of the material.
Electrodes, placed at the surfaces of an electret and temporarily shorted, will transfer electrons around the shorting circuit to exactly neutralize the electret's external field. Once this occurs, no signal is detectable from the undisturbed system. If, however, an event occurs which either reorients some of the trapped dipoles or frees a portion of the trapped charges, the electret field is no longer exactly neutralized and a detectable voltage difference will appear between the electrodes. The more effective the event is at destroying the electret field, the greater the observed voltage.
Electrets are currently used in several radiation detection arrangements. In one of these arrangements, the electrostatic field from the electret is used to replace the high voltage power supply in an ionization chamber. In another arrangement of this type, as reported by M. Ikiya and T. Imki in Health Physics Volume 39, page 797, November, 1980, an electret supported charge multiplication arrangement is disclosed. In another example of this type, as disclosed in the patent of S. Hellstom, U.S. Pat. No. 3,949,178, an electret which is maintained in a stable voltage condition through the use of radioactive material excitation is used in a microphone and in other devices. Electrets are also desirably used in dosimeters where their simplicity and the absence of electret displaced electronic apparatus is of significant advantage. In such dosimeter devices the electret is usually received in a gas chamber where ionizing radiation produces electron-ion pairs in the gas. These pairs are then separated by the electric field of the electret. The collection of electrons or ions on the electret surface, however, tends to depolarize such electrets and they must be maintained by recharging or by arrangements such as is disclosed in the Hellstrom patent above.
Other known electret dosimeter arrangements use the gross depolarization of an electret by direct flux interaction to accomplish radiation measurements. Such devices are suitable for large or megaroentgen radiation doses but are generally considered too insensitive for use as personal dosimeter instruments. In such dosimeter arrangements, the degree of electret discharge by the radiation interaction is measured by comparing the electric field strength at a fixed position above the electret to that observed before the irradiation event. Alternately, a thermally simulated discharge, wherein current flow between surface electrodes of the electret is measured during a slow heating sequence, may be used to measure the degree of electret discharge. The Patent of P. E. Homart, U.S. Pat. No. 2,663,802 describes a neutron detector in which reliance on this gross depolarization concept is used.
Electrets have also been used in electrostatic particle traps that are employed for radon daughter dosimetry purposes. In this arrangement the electret is used as an electrostatic precipitator so that dust particles bearing radon daughters are attracted to and entrapped on the electret charged surface. In such instruments, the electret provides dust collection of the electrostatic precipitate without requiring a high voltage power supply.
The patent art reveals the prior use of fissionable materials such as uranium in a variety of applications including the neutron stimulated generation of fission fragments. In such uses uranium is often formed into the configuration of a foil to enable its bombardment by thermal neutrons. Such bombardment can generate fission fragments that are employed for a variety of useful purposes, such as the acceleration of deuterium and tritium particles to generate higher energy neutrons, for example. One apparatus of this type is shown in the Patent of L. G. Miller et al, U.S. Pat. No. 3,976,888.
The actual fabrication of electret devices is, as is implied above, known in a number of variations in the art. One example of electret formation is moreover to be found in the Patent of S. W. Sapieha. U.S. Pat. No. 4,407,852.
Electrets therefore have found several uses in dosimetry and other forms of radiation detection, however, the use of electrets in sensitive neutron flux instruments and in the detection of individual radiation events appears not to have been practiced. It is notable in this regard that the absence of electrical charge in a neutron makes direct neutron detection somewhat difficult to accomplish. It is parenthetically notable, however, that a sufficiently energetic radiation event such as the interaction of a fission fragment with an electret material can be detected directly as a change in field strength at the surface of the electret.
As a result of the absence of more readily detected charge, therefore, the detection of neutrons is usually accomplished through recognition of one or more secondary effects of a neutron interaction rather than by direct detection of the neutron particle itself. The principal methods of neutron detection used in present day apparatus are therefore classifiable as detectors relying upon neutron induced transmutations with detectable products, neutron activation of target nuclei and elastic scattering of neutrons with detection of the recoil product.
Neutron-induced transmutations involve such reactions as neutron generation of alpha particles (n,alpha), as well as (n,gamma), (n,p), and (n,fission) detection arrangements. An example of the (n,alpba) type detector is the boron trifluoride gas chamber wherein boron-10 undergoes an (n,alpha) reaction when it absorbs a thermal neutron. This reaction has a cross-section of about 1000 barns and yields 2.78 million electron volts. For this type of neutron detector, gaseous boron trifluoride is placed in an ionization chamber where the resulting alpha particle is readily detected. A similar detector using the (n,p) reaction is found in the helium-3 gas chamber detector. In this detector helium-3 undergoes an (n,p) reaction with a cross section of 5,400 barns and an energy yield of 675 keV. The energy (divided between a tritium atom and a proton) ionizes krypton gas in a chamber to produce the detectable signal. Both the boron and helium gas chamber detectors provide good efficiency and sincerity but are sensitive to other forms of radiation, particularly gamma radiation.
The fission track dosimeter is an example of the (n,fission) reaction. In this instrument a detector material consisting of a layer of mica or special plastic is placed next to a layer of uranium enriched in uranium-235. Fission events in the uranium-235 drive fission fragments into the detector material producing damage tracks. After appropriate chemical treatment, these damage tracks become visible and may be counted under a microscope. The number of damage tracks is, of course, related to the total thermal neutron fluence. The track detector makes a practical personnel dosimeter but its readout requires costly and time-consuming chemical treatment and optical counting.
Neutron detection by activation differs from transmutation detection in that the activation product has a reasonably long half-life before producing detectable products while transmutation produces detectable products almost immediately. The use of activation foils is a prime example of this detection method. In these detectors one of several elements, including indium, gold, silver, vanadium, and others, absorbs neutrons to produce radioactive isotopes. Foils of these materials are placed in a neutron flux and the resulting foil activity is then used to determine the flux intensity level. Since many of these materials have an effective neutron energy threshold for activation, the use of several different foils also yields neutron energy spectrum data. A drawback of activation analysis is, however, the high radiation fluences required for accurate results.
Neutron detectors using elastic scattering are almost exclusively proton recoil devices. When fast neutrons interact with a hydrogenous material, hydrogen nuclei are struck and absorb up to 100 percent of the neutron's energy. An ionization chamber filled with, for example, a hydrocarbon gas and lined with polyethylene can therefore measure an absorbed dose and also effectively rejects gamma energy. This type of detector can be fairly neutron-specific and sensitive but is not useful for detecting thermal neutrons.
For low-level thermal neutron fluxes, another detector, a semiconductor doped with boron-10, helium-3, or lithium-6 provides reasonable efficiency but at the cost of physical and electronic complexity, as well as, sensitivity to other forms of radiation.
Each of these types of neutron detectors therefore have certain advantages and disadvantages. Fission track dosimeters are small, simple and fairly sensitive but measure only total fluence and cannot be read without complex processing. Activation detectors require high flux levels; elastic scattering detectors are not useful for low energy thermal neutrons; and semiconductor detectors involve complex electronics. There is clearly, therefore, a need in the art for a low-level thermal neutron detector that is simple, compact, neutron-specific, and capable of real-time readout.